Which of these numbers is prime? ${5,\ 15,\ 85,\ 91,\ 96}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 5 are 1 and 5. The factors of 15 are 1, 3, 5, and 15. The factors of 85 are 1, 5, 17, and 85. The factors of 91 are 1, 7, 13, and 91. The factors of 96 are 1, 2, 3, 4, 6, 8, 12, 16, 24, 32, 48, and 96. Thus, 5 is a prime number.